1. Field of the Invention
This invention relates to a measuring instrument and a measuring method, in which inner and outer dimension or a diameter of an object is measured by the distance between a pair of probes when the pair of the probes is abutted to the object to be measured while moving relatively; and to a probe for a measuring instrument that is most suitable for the inner dimension measurement.
2. Description of the Related Art
An inner and outer dimension measuring instrument, in which inner and outer dimension of an object is measured by the distance between a pair of probes when the probes are abutted to a surface of the object while moving relatively, is used.
According to the inner and outer dimension measuring instrument, the inner and outer dimension of the object is measured by the distance between the pair of the probes, so that it is possible to measure various sizes of objects, such as the diameters of a ring-shaped object, a diameter of a hole or the width of a groove which is formed on an object, and a thickness of an object.
Conventionally, for example, an inner and outer dimension measuring instrument 110, shown in FIG. 20, is known as an inner and outer dimension measuring instrument.
The inner and outer dimension measuring instrument 110 is composed of: a body 111; a table 112 mounted on the body 111 so that an object to be measured W is put thereon; and a pair of probes P1 and P2 extending upward from the surface of the table 112 on which the object W is put, and the probes respectively moved in respectively opposite directions A and B to abut to and separate from surfaces of the object W.
The body 111 is provided therein with a rail 115 extending along a moving direction of the probes P1, P2, and the pair of sliders 55 movably supported with respect to the rail 115 and respectively supporting the bottom ends of a pair of probes P1 and. P2. The table 112 is provided on the upper face of the body 111 and has an X-Y table 113 to adjust the horizontal position of the object W, and a tilt adjusting table 114 to adjust a tilt of the object W against the probes P1 and P2.
For example, in the measurement of the inner dimension of the object W, after the probes P1 and P2 approach each other, the object W is put on the table 112. After that, the probes P1 and P2 are respectively moved in the directions A opposite to each other. While the probes P1 and P2 are abutted to the inner side of the object W, each position of the sliders 116 is read from a displacement detector (not shown), thus measuring the inner diameter of the object W as the distance between the pair of the probes P1 and P2. Note that, in the measurement, force acting in a direction toward the object W is applied to the probes P1 and P2 through the sliders 116, so that the contacting force is maintained between the object W and the probes P1 and P2. As a result, the probes P1 and P2 can be surely abutted to the inner circumferential face of the object W.
Another inner and outer dimension measuring instrument 120, shown in FIG. 21, is known.
The inner and outer dimension measuring instrument 120 is composed of: a body 121, a table 122 mounted on the body 121, a pair of rails 125 provided on the body 121 on both sides of the table 122, a slider 126 provided on each of a pair of the rails 125, an arm 127 attached to each slider 126, and a pair of probes P1 and P2 respectively provided to the ends of the arms 127. Similarly to the aforementioned table 112, the table 122 has an X-Y table 123 and a tilt adjusting table 124.
Similarly to the aforementioned inner and outer dimension measuring instrument 110, for example, in the measurement of the inner dimension of the object W, after the probes P1 and P2 are moved in the directions A to separate from each other, each position of the sliders 126 is read from a displacement detector (not shown) when the probes P1 and P2 are abutted to the inner side-face of the object W, thus measuring the inner diameter of the object W as a distance between the probes P1 and P2.
The following disadvantages, however, can be listed when the objects W having various sizes are measured with high accuracy by the aforementioned inner and outer dimension measuring instruments 110 or 120.
Disadvantage 1
In the inner and outer dimension measuring instrument 110, the table 112 has a multilayer structure composed of the X-Y table 113 and the tilt adjusting table 114, so that each of the probes P1 and P2 should have a sufficient length to project above the table 112 from the slider 116 located under the table 112. In consequence, when the probes P1 and P2 are abutted to the inner side-face of the object W at the predetermined contacting force, a difference may be occurred between the actual distance between the probes P1 and P2 and a measured value indicated on the displacement detector.
On the inner and outer dimension measuring instrument 120, although the length of the probes P1 and P2 is shorter, the length of the arm 127 needs to be longer in order to measure any object W having various sizes. According to the extent of the length of the arm 127, the arm 127 is flexed by the dead weight thereof and the contacting force of the probes P1 and P2 to the object W, thereby also producing a difference between the actual distance between the pair of the probes P1 and P2 and a measured value indicated on the displacement detector.
To resolve the aforementioned measuring errors, in the inner and outer dimension measuring instruments 110 and 120, the measurement of the object W is carried out with a relative measurement in which a measured value is corrected with pre-measured standard sample.
For the reason that the flexure of the probes P1 and P2 or the arm 127 exerts an influence upon the measurement in these conventional inner and outer dimension measuring instruments 110 and 120, the relative measurement is always needed irrespective of the required accuracy of the measurement of the object W, resulting in complicated processes for measuring the object W.
In the measurement using the longer probes P1 and P2 and the longer arm 127, the absolute amount of the flexure is larger, so that the dispersion in the contacting force of the probes P1 and P2 exerts an influence upon the dispersion in the measured values, resulting in difficulties in the measurement with high accuracy.
Disadvantage 2
To properly measure the inner or the outer diameter of the object W by any one of the aforementioned inner and outer dimension measuring instruments 110 and 120, after the object W is put on the inner and outer dimension measuring instrument, it is needed to perform two adjustments, that is, adjustment for aligning a measurement axis (an axis line on which the probe is moved) of the measuring instrument with a diametral position of the object W (a measuring position adjustment), and adjustment for correcting a tilt of the axis of the inner or outer diameter of the object W relative to an axis line perpendicular to the measurement axis (a tilt adjustment). In other words, the axis of the inner or outer diameter of the object W should be adjusted to intersect at right angles to the measurement axis.
For example, in the measurement of the inner diameter, as shown in FIG. 22, when the diametral position of the object W differs from the measurement axis of the measuring instrument, a space d between the pair of the probes P1 and P2 is smaller than a diameter D of the object W, so that the accurate measurement is impossible. As shown in FIG. 23, in order to properly measure the inner diameter (or the outer diameter) of the object W, the diametral position of the object W is needed to be aligned with the measurement axis of the measuring instrument.
As shown in FIG. 24, when the axis of the inner diameter (or the outer diameter) of the object W has a tilt with respect to the axis line perpendicular to the measurement axis, the space d between the pair of the probes P1 and P2 is larger than the diameter D of the object W, so that the accurate measurement is also impossible. As shown in FIG. 25, the tilt of the axis of the inner diameter (or the outer diameter) of the object W with respect to the axis line perpendicular to the measurement axis should be corrected in order to properly measure the inner (or outer) diameter of the object W.
It is laborious and takes much time to perform an adjusting process to align the measurement axis of the measuring instrument with the diametral position of the object W and an adjusting process to correct a tilt of the axis of the inner or outer diameter of the object W with respect to an axis line perpendicular to the measurement axis.
For example, in the measuring position adjustment, after the probes P1 and P2 are abutted to the inner circumferential face of the object W, while the pair of the probes P1 and P2 and the object W are relatively moved in a perpendicular direction (a diametrical direction) with respect to the measurement axis, the space between the pair of the probes P1 and P2 is measured to find positions where the the probes P1 and P2 are at the maximum distance from each other. By positioning the probes P1 and P2 and the object W at the found position, the diametral position of the object W and the measurement axis of the measuring instrument are aligned. In this case, the moving error element in a direction of the measurement axis has little effect, so that the straight moving precision of a relative movement mechanism is not required to be especially high.
On the other hand, where the tilt adjustment for correcting the tilt of the axis of the inner or outer diameter of the object W with respect to the axis line perpendicular to the measurement axis is concerned, even when the pair of the probes P1 and P2 and the object W are relatively moved in the axial direction perpendicular to the measurement axis (the axis direction of the inner or outer diameter), with the pair of the probes PI and P2 being abutted to the inner or outer circumferential face of the object W, the pair of the probes P1 and P2 moves in the same direction as the direction of the measurement axis, so that the detection of the tilt is impossible. In consequence, this adjustment needs great effort and a lot of time.
It should be mentioned that, in the measuring position adjustment in an inner-diameter measuring instrument for measuring an inner diameter with a single probe, after the probe is abutted to the inner circumferential face of an object, displacement of the probe is detected to find a position where the displacement of the probe is maximum (or minimum) while the probe and the object are being relatively moved in a perpendicular direction (a diametrical direction) with respect to a measurement axis. After that the probe and the object are positioned at the found position. In this case, the movement straightness when the probe and the object are relatively moved in the perpendicular direction with respect to the measurement axis exerts an influence upon a measured value. That is, when the moving error element in a direction of the measurement axis is in a relative movement mechanism, the moving error element is added to the measured value, so that the straight moving precision of a relative movement mechanism is required to be high.
Disadvantage 3
As shown in FIG. 26, each of the probes P1 and P2 of both inner and outer dimension measuring instruments 110 and 120 described above is composed of a cylindrical stick-shaped steel shaft 131 connected to a slider at the base end thereof, and a ball-shaped steel contact portion brazing-welded on the distal end of the shaft 131, the outer circumferential face of the contact portion 132 being projected from the circumferential face of the shaft 131 in a diametrical direction so as to be abutted to the object W.
When the probes P1 and P2 are moved in directions opposite to each other and each contact portion 132 of the probes P1 and P2 is abutted to the face of the object W, the dimension measurement is performed in a state that the contacting force is applied to the probes P1 and P2 so that the contact portion 132 is in reliable contact with the face of the object W. Therefore, flexural deformation is produced on the shaft 131 by repulsion F from the face to be measured of the object W, whereby a difference between an actual value of the diameter and an indicated value on the displacement detector is produced.
For this reason, it is understood that the accurate measurement for L1 is reduced due to the larger amount of flexure when the rigidity of the shaft 131 is insufficient. In consequence, in order to ensure accuracy of the measurement, the diameter of the shaft 131 should be increased so as to increase the rigidity thereof and minimize the flexure thereof.
However, as shown in FIGS. 27A and 27B, in measuring inner diameter L2 of a small hole Wh formed on the object W, two probes P1 and P2 are simultaneously inserted into the small hole Wh. In this case, the diameter of each shaft 131 of the probes P1 and P2 should not be increased so much.
As shown in FIGS. 28A and 28B, a shaft 141 of an approximately semi-circular cross-section is formed by dividing a circular cross-section of a shaft, which is smaller than a diameter of the small hole Wh, at an axis line D perpendicular to the moving axis C. A hemispherical contact portion 142 is provided on the end of the shaft 141. Thus, the sections of shaft 141 of probes P1 and P2 can be increased.
However, even in the aforementioned probes P1 and P2, an area along the moving axis C in which repulsion F acts is still unenlarged, so that the rigidity relative to a force in a direction of the moving axis is not obtained sufficiently, resulting in the difficulties of the high accurate measurement for the inner dimensions L1 and L2 by the inner and outer dimension measuring instruments 110 and 120.
Moreover, in the probes P1 and P2, the shaft 141 has residual distortion depending on the size of repulsion F and a measured dimension can vary, therefore it is difficult to obtain reliable results of the measurement.